1. Field of Invention
This invention relates to an electricity to air pressure converter for converting an electrical signal into an air pressure corresponding thereto; and, more particularly, to such a converter having improved stability, compactness and is inexpensive.
2. Description of the Prior Art
FIGS. 1(A) and 1(B) show a conventional electricity/air pressure converter, such as disclosed, for example, in Japan Unexamined Patent No. 4-73,401; wherein FIG. 1(A) is a longitudinal sectional view and FIG. 1(B) is a right side view. A U-shaped permanent magnet 10 is shown comprising an upper leg portion 11, which is an N-pole, and a lower leg portion 12, which is a S-pole. Sides 15 and 16, of a pair of U-shaped yokes 13 and 14, made of a soft magnetic material, are secured to leg portions 11 and 12, respectively, with a constant space L.sub.M maintained therebetween. Accordingly, the leg portions of yokes 13 and 14 define spaces A.sub.1 and A.sub.2, which are equal to each other.
A movable piece 17, made of a soft magnetic material, extends through spaces A.sub.1 and A.sub.2 in the middle thereof. Movable piece 17 is secured to side 15 of yoke 13, in the vicinity of one end thereof, by a spring member 18, made of a non-magnetic material, such as beryllium copper, to be rotatable about this point. Movable piece 17 is configured to be asymmetric about the center thereof and is dynamically off balance. Thus, a counter weight 19 is secured to one end of yoke 13. A coil 20 is disposed in an internal space defined by the pair of U-shaped yokes 13 and 14 facing each other so that coil 20 surrounds movable piece 17 with a space maintained therebetween.
One side of the other end of movable piece 17 is secured by a securing pin 22, through a spring member 21, and a nozzle 23 is disposed to face the other side of this end of movable piece 17 with a small gap therebetween.
Nozzle 23 is supplied with a supply air pressure P.sub.s through a restrictor 24, and an output air pressure P.sub.o is obtained as a back pressure of nozzle 23 between restrictor 24 and nozzle 23 through a conduit 25.
The operation of the just described converter will now be discussed with reference to FIG. 2. In space A.sub.1, an air gap length L.sub.g11 is formed between one leg portion of yoke 13 and movable piece 17; and an air gap length L.sub.g12 is formed between one leg portion of yoke 14 and movable piece 17. In space A.sub.2, an air gap length L.sub.g21 is formed between the other leg portion of yoke 13 and movable piece 17; and an air gap length L.sub.g22 is formed between the other leg portion of yoke 14 and movable piece 17.
In each of the air gaps A.sub.g11, A.sub.g12, A.sub.g21, and A.sub.g22, defined by air gap lengths L.sub.g11, L.sub.g12, L.sub.g21, and L.sub.g22, there exists magnetic flux .phi..sub.E which is generated by magnetic flux .phi..sub.d, generated by permanent magnet 10, and an input current I flowing through coil 20.
Magnetic flux .phi..sub.d flows from the N-pole of permanent magnet 10 to the S-pole thereof through each of air gaps A.sub.g11, A.sub.g12, A.sub.g21, and A.sub.g22, while magnetic flux .phi..sub.E is the sum of, for example, magnetic flux flowing through air gaps A.sub.g11 and A.sub.g12 and movable piece 17 and magnetic flux flowing through air gaps A.sub.g21 and A.sub.g22 and movable piece 17, as indicated by the arrow.
Thus, if the input current I is zero, the magnetic fields formed in air gaps A.sub.g21 and A.sub.g22 are equal in strength because the fields have only magnetic flux .phi..sub.d generated by permanent magnet 10. Since a magnetic attracting force in an air gap is proportional to the square of the strength of the magnetic field, the movable piece 17 is disposed in the middle of space L.sub.M.
On the other hand, since movable piece 17 is secured at a point in the vicinity of one end thereof by spring member 18, movable piece 17 moves about this point, which serves as a support point. Thus, as input current I flows through coil 20, magnetic flux .phi..sub.E generated by input current I, is added to magnetic flux .phi..sub.d (.phi..sub.d +.phi..sub.E) in air gap A.sub.g21 while it is subtracted from magnetic flux .phi..sub.d (.phi..sub.d -.phi..sub.E) in air gap A.sub.g22. As a result, an upward force F.sub.2 acts on movable piece 17 in the air gap, as depicted.
Since air gap A.sub.g11 has magnetic flux (.phi..sub.d -.phi..sub.E) and air gap A.sub.g12 has magnetic flux (.phi..sub.d +.phi..sub.E), a downward force F.sub.1 acts on movable piece 17 in the air gap, which is closer to the support point, as depicted.
Since the lengths of air gaps L.sub.g11, L.sub.g12, L.sub.g21, and L.sub.g22, are equal, forces F.sub.1 and F.sub.2 are equal in magnitude and opposite each other in direction. The force in each air gap is proportional to the square of the magnetic flux therein. Thus, the following equation is derived: EQU F.sub.1 =F.sub.2 .alpha.(.phi..sub.d +.phi..sub.E).sup.2 -(.phi..sub.d -.phi..sub.E).sup.2 ( 1)
The forces generated can be expressed as follows: EQU F.sub.1 =F.sub.2 =K.sub.M .phi..sub.d .phi..sub.E ( 2)
wherein K.sub.M represents a proportion constant.
Although movable piece 17 is slightly displaced when input current I flows, the displacement is ignored because it is normally on the order of 5 to 50 .mu.m and L.sub.g is normally on the order of 1 to 2 mm.
As a result, rotational torque T, as given by below equation 3, is generated on movable piece 17, counterclockwise, about spring member 18 to move movable piece 17 toward yoke 13. EQU T=F.sub.2 .multidot.L.sub.2 -F.sub.1 .multidot.L.sub.1 =F(L.sub.2 -L.sub.1)=F.multidot.L (3)
wherein F=F.sub.1 =F.sub.2 and L=L.sub.2 =L.sub.1.
As movable piece 17 moves toward yoke 13, the space between nozzle 23 and movable piece 17 expands, thereby reducing the back pressure of the nozzle, and consequently, reducing output air pressure P.sub.o. Thus, an output air pressure P.sub.o, corresponding to the input current I, is obtained.
However, the described conventional converter has various problems, such as listed below.
1. One problem is that torque T which is generated, is expressed by the following equation: EQU T=K.sub.M .multidot..phi..sub.E .multidot..phi..sub.d .multidot.L (4)
Furthermore, magnetic flux .phi..sub.E generated by coil 20 can be expressed as follows: ##EQU1## wherein RL.sub.g represents the magnetic resistance of the air gap; R.sub.yo represents the magnetic resistance of yoke 13; R.sub.m represents the magnetic resistance of movable piece 17; and N represents the number of turns of coil 20.
N and RL.sub.g are constant while the magnetic resistance R.sub.m of movable piece 17 and magnetic resistance R.sub.yo of yoke 13 includes some error relative to the input current I because of the non-linearity and hysteresis properties of the soft magnetic materials used in the components.
Since magnetic flux .phi..sub.E, generated by coil 20, flows through movable piece 17 in the direction indicated by the dashed line in FIG. 2, i.e. in a direction parallel to the central axis of coil 20, the magnetic resistance R.sub.m of movable piece 17 is increased as the sectional area S (=t.multidot.w) of movable piece 17, in a direction perpendicular to the direction of the flow, is decreased.
As a result, the ratio of magnetic resistance R.sub.m to magnetic flux .phi..sub.E is increased, as is apparent from above equation (5). This increases the influence of non-linearity and hysteresis of movable piece 17, which is made of a soft magnetic material, thereby increas-in the error in the generated torque relative to the input current I.
If the sectional area S is decreased below a predetermined value, magnetic flux density B.sub.E =.phi..sub.E /S approaches the saturation magnetic flux density of the soft magnetic material. As a result, the linear relationship between the input current I and the output torque T is completely lost.
Furthermore, if width w of movable piece 17 is decreased, magnetic resistance RL.sub.g is increased at each of the air gaps. If the magnetomotive forces of coil 20 and permanent magnet 10 are equal, this results in decreases in .phi..sub.E and .phi..sub.d, thereby reducing conversion efficiency (i.e. generated toque divided by input current). Accordingly, vulnerability to disturbances is increased and stability is reduced.
Accordingly, the sectional area S of movable piece 17 cannot be decreased below a predetermined amount, and it is thus difficult to reduce the mass of the parts below a predetermined amount. Thus, the particular part discussed cannot be reduced in overall size. Also, there is poor anti-vibration and anti-shock properties.
In addition, since the resonance frequency cannot be increased beyond a predetermined value, and a movable portion having a large inertia is controlled by a force smaller than a predetermined value, the part has poor controllability and is difficult to operate in a stable manner.
2. Another problem is that many magnetic paths, through which leakage flux flows,/which are formed around each of the air gaps and at the sides of yokes 13 and 14, other than at the air gaps, and the converter, are not entirely covered by a magnetic material. Thus, the output of the converter can be adversely affected by any external magnetic material placed in the vicinity thereof, or by any magnetic field which exists externally of the converter.
To cover the entire converter with a magnetic material in order to magnetically shield the converter and thus reduce such aforementioned effect, the magnetic material must be spaced sufficiently away from the converter so that it does not affect the magnetic flux at the air gaps. This would increase the overall size of the converter and the number of components, thereby making it difficult to provide a converter which is compact in size, inexpensive, and stable in operation against magnetic disturbances.
3. A further problem is that movable piece 17 is secured to yoke 13 through spring member 18 and rotates about such support point. Spring member 21 and securing pin 22 are provided on the other end of movable piece 17 in order to maintain the overall spring constant above a predetermined value and to adjust the position of movable piece 17 relative to the pair of yokes 13 and 14.
It is necessary to fabricate movable piece 17 and yokes 13 and 14 using soft magnetic material, and spring memebers 18 and 21 using non-magnetic materials, such as beryllium copper or materials which have some magnetism but with low magnetic permeability and a small coercive force, such as spring type materials of an austenited stainless steel.
Securing points, wherein materials of different types are secured together, are required between spring member 18 and movable piece 17, between spring member 18 and yokes 13 and 14, and between movable piece 17 and spring member 21. Any positional shift in such points can significantly and adversely affect the stability of the output because such a shift produces disturbance torque directly to movable piece 17 and the direction of the shift coincides with the direction in which nozzle 23 is displaced.
Although it is desirable to use a reliable method, such as welding, to secure such parts, screwing or soldering is generally used because different types of materials are secured together, and that any thermal effect which can deteriorate spring properties and magnetic properties, must be avoided. However, since different types of materials having different thermal expansion coefficients are involved, it is difficult to maintain high stability because positional shifts can be easily caused by temperature changes.
4. A still further problem is that since the pair of yokes 13 and 14, movable piece 17, and spring members 18 and 21, are separate components, it is difficult to accurately position these parts. It is important to achieve positional accuracy in the vertical direction, that is distances L.sub.g11, L.sub.g12, L.sub.g21, and L.sub.g22 between yokes 13 and 14 and movable piece 17 should be equal.
However, yokes 13 and 14 are disposed with a space therebeween in which movable piece 17 is rotatably interposed using spring members 18 and 19, serving as support points. It is thus difficult to define the positional relationship between the components based only on the accuracy achieved during processing. This results in an increase in the time required for assembly of the components, and consequently, cost of manufacture is increased.
5. Another problem is that since movable piece 17 cannot be reduced in size below a predetermined amount, and since piece 17 rotates about a support point, a counter weight 19 must be provided to balance the two ends thereof about the support point. This increases the overall size of the device, thus making it difficult to fabricate the device at a low cost.
This problem exists also in the commonly used force balance type of electricity to air pressure converter, wherein an input current of 4 to 20 mA is directly inputted to the coil, or an electricity to air pressure converter of an electropneumatic positioner.
Taking the electropneumatic positioner, and utilizing an electricity to air pressure converter as described above as a component, as an example, the problem is compounded in the conventional device of FIG. 3.
In FIG. 3, a current signal I.sub.o of 4 to 20 mA or a digital signal, such as a field bus, is inputted to an input/power circuit 26, which provides a circuit power supply using current signal I.sub.o and which provides a control voltage V.sub.c, which is proportional to current signal I.sub.o, and outputs the voltage V.sub.c to a control arithmetic circuit 27.
Control arithmetic circuit 27 receives control voltage V.sub.c and a feedback signal V.sub.f outputted by valve displacement sensor 28, comprising a potentiometer or the like, and generates a signal which represents the difference between the two signals which is then outputted to an electricity/air pressure converter 29.
A supply air pressure P.sub.s is supplied to electricity/air pressure converter 29, which may be the converter shown in FIGS. 1(A) and 1(B), and as a nozzle back pressure, supply air pressure P.sub.s is supplied to and amplified by a pilot relay 30 and is outputted to control valve 31 as an air pressure signal P.sub.o.
Control valve 31 is driven according to the air pressure signal P.sub.o, and the displacement of the stem, which defines the opening of the valve, is detected as the displacement of the valve, by valve displacement sensor 28, and is fed back,as negative feedback, to control arithmetic circuit 27, as feedback signal V.sub.f.
Thus, control arithmetic circuit 27 performs a control function so that the control voltage V.sub.c and feedback signal V.sub.f agree with each other,thereby producing a valve opening which is proportional to the current signal I.sub.o.
The feedback signal V.sub.f is fed back as an electrical signal, and current is consumed also by control arithmetic circuit 27, and valve displacement sensor 28 and the like. Thus, there is a limit to the amount of current that can be distributed to the converter 29.
As a result, the FIG. 3 device has a problem of abnormality in output caused by fluctuations due to disturbances which cannot be corrected, especially when the input current is small, such as about 4 mA.
Thus, it can be appreciated that the prior art leaves room for improvement.